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RFC 1771 - 4.3 UPDATE Message Format

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### 4.3 UPDATE Message Format

UPDATE messages are used to transfer routing information between BGP peers. The information in the UPDATE packet can be used to construct a graph describing the relationships of the various Autonomous Systems. By applying rules to be discussed, routing information loops and some other anomalies may be detected and removed from inter-AS routing.

An UPDATE message is used to advertise a single feasible route to a peer, or to withdraw multiple unfeasible routes from service (see 3.1). An UPDATE message may simultaneously advertise a feasible route and withdraw multiple unfeasible routes from service. The UPDATE message always includes the fixed-size BGP header, and can optionally include the other fields as shown below:

          +-----------------------------------------------------+
          |   Unfeasible Routes Length (2 octets)               |
          +-----------------------------------------------------+
          |  Withdrawn Routes (variable)                        |
          +-----------------------------------------------------+
          |   Total Path Attribute Length (2 octets)            |
          +-----------------------------------------------------+
          |    Path Attributes (variable)                       |
          +-----------------------------------------------------+
          |   Network Layer Reachability Information (variable) |
          +-----------------------------------------------------+

          Unfeasible Routes Length:

             This 2-octets unsigned integer indicates the total length of
             the Withdrawn Routes field in octets.  Its value must allow the
             length of the Network Layer Reachability Information field to
             be determined as specified below.

             A value of 0 indicates that no routes are being withdrawn from
             service, and that the WITHDRAWN ROUTES field is not present in
             this UPDATE message.

          Withdrawn Routes:

             This is a variable length field that contains a list of IP
             address prefixes for the routes that are being withdrawn from
             service.  Each IP address prefix is encoded as a 2-tuple of the
             form <length, prefix>, whose fields are described below:

                      +---------------------------+
                      |   Length (1 octet)        |
                      +---------------------------+
                      |   Prefix (variable)       |
                      +---------------------------+

             The use and the meaning of these fields are as follows:

             a) Length:

                The Length field indicates the length in bits of the IP
                address prefix. A length of zero indicates a prefix that
                matches all IP addresses (with prefix, itself, of zero
                octets).

             b) Prefix:

                The Prefix field contains IP address prefixes followed by
                enough trailing bits to make the end of the field fall on an
                octet boundary. Note that the value of trailing bits is
                irrelevant.

          Total Path Attribute Length:

             This 2-octet unsigned integer indicates the total length of the
             Path Attributes field in octets.  Its value must allow the
             length of the Network Layer Reachability field to be determined
             as specified below.

             A value of 0 indicates that no Network Layer Reachability
             Information field is present in this UPDATE message.

          Path Attributes:

             A variable length sequence of path attributes is present in
             every UPDATE.  Each path attribute is a triple <attribute type,
             attribute length, attribute value> of variable length.

             Attribute Type is a two-octet field that consists of the
             Attribute Flags octet followed by the Attribute Type Code
             octet.

                    0                   1
                    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |  Attr. Flags  |Attr. Type Code|
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             The high-order bit (bit 0) of the Attribute Flags octet is the
             Optional bit.  It defines whether the attribute is optional (if
             set to 1) or well-known (if set to 0).

             The second high-order bit (bit 1) of the Attribute Flags octet
             is the Transitive bit.  It defines whether an optional
             attribute is transitive (if set to 1) or non-transitive (if set
             to 0).  For well-known attributes, the Transitive bit must be
             set to 1.  (See Section 5 for a discussion of transitive
             attributes.)

             The third high-order bit (bit 2) of the Attribute Flags octet
             is the Partial bit.  It defines whether the information
             contained in the optional transitive attribute is partial (if
             set to 1) or complete (if set to 0).  For well-known attributes
             and for optional non-transitive attributes the Partial bit must
             be set to 0.

             The fourth high-order bit (bit 3) of the Attribute Flags octet
             is the Extended Length bit.  It defines whether the Attribute
             Length is one octet (if set to 0) or two octets (if set to 1).
             Extended Length may be used only if the length of the attribute
             value is greater than 255 octets.

             The lower-order four bits of the Attribute Flags octet are .
             unused. They must be zero (and must be ignored when received).

             The Attribute Type Code octet contains the Attribute Type Code.
             Currently defined Attribute Type Codes are discussed in Section
             5.

             If the Extended Length bit of the Attribute Flags octet is set
             to 0, the third octet of the Path Attribute contains the length
             of the attribute data in octets.

             If the Extended Length bit of the Attribute Flags octet is set
             to 1, then the third and the fourth octets of the path
             attribute contain the length of the attribute data in octets.

             The remaining octets of the Path Attribute represent the
             attribute value and are interpreted according to the Attribute
             Flags and the Attribute Type Code. The supported Attribute Type
             Codes, their attribute values and uses are the following:

             a)   ORIGIN (Type Code 1):

                ORIGIN is a well-known mandatory attribute that defines the
                origin of the path information.   The data octet can assume
                the following values:

                      Value      Meaning

                      0         IGP - Network Layer Reachability Information
                                   is interior to the originating AS

                      1         EGP - Network Layer Reachability Information
                                   learned via EGP

                      2         INCOMPLETE - Network Layer Reachability
                                   Information learned by some other means

                Its usage is defined in 5.1.1

             b) AS_PATH (Type Code 2):

                AS_PATH is a well-known mandatory attribute that is composed
                of a sequence of AS path segments. Each AS path segment is
                represented by a triple <path segment type, path segment
                length, path segment value>.

                The path segment type is a 1-octet long field with the
                following values defined:

                      Value      Segment Type

                      1         AS_SET: unordered set of ASs a route in the
                                   UPDATE message has traversed

                      2         AS_SEQUENCE: ordered set of ASs a route in
                                   the UPDATE message has traversed

                The path segment length is a 1-octet long field containing
                the number of ASs in the path segment value field.

                The path segment value field contains one or more AS
                numbers, each encoded as a 2-octets long field.

                Usage of this attribute is defined in 5.1.2.

             c)   NEXT_HOP (Type Code 3):

                This is a well-known mandatory attribute that defines the IP
                address of the border router that should be used as the next
                hop to the destinations listed in the Network Layer
                Reachability field of the UPDATE message.

                Usage of this attribute is defined in 5.1.3.

             d) MULTI_EXIT_DISC (Type Code 4):

                This is an optional non-transitive attribute that is a four
                octet non-negative integer. The value of this attribute may
                be used by a BGP speaker's decision process to discriminate
                among multiple exit points to a neighboring autonomous
                system.

                Its usage is defined in 5.1.4.

             e) LOCAL_PREF (Type Code 5):

                LOCAL_PREF is a well-known discretionary attribute that is a
                four octet non-negative integer. It is used by a BGP speaker
                to inform other BGP speakers in its own autonomous system of
                the originating speaker's degree of preference for an
                advertised route. Usage of this attribute is described in
                5.1.5.

             f) ATOMIC_AGGREGATE (Type Code 6)

                ATOMIC_AGGREGATE is a well-known discretionary attribute of
                length 0. It is used by a BGP speaker to inform other BGP
                speakers that the local system selected a less specific
                route without selecting a more specific route which is
                included in it. Usage of this attribute is described in
                5.1.6.

             g) AGGREGATOR (Type Code 7)

                AGGREGATOR is an optional transitive attribute of length 6.
                The attribute contains the last AS number that formed the
                aggregate route (encoded as 2 octets), followed by the IP
                address of the BGP speaker that formed the aggregate route
                (encoded as 4 octets).  Usage of this attribute is described
                in 5.1.7

          Network Layer Reachability Information:

             This variable length field contains a list of IP address
             prefixes.  The length in octets of the Network Layer
             Reachability Information is not encoded explicitly, but can be
             calculated as:

                UPDATE message Length - 23 - Total Path Attributes Length -
                Unfeasible Routes Length

             where UPDATE message Length is the value encoded in the fixed-
             size BGP header, Total Path Attribute Length and Unfeasible
             Routes Length  are the values encoded in the variable part of
             the UPDATE message, and 23 is a combined length of the fixed-
             size BGP header, the Total Path Attribute Length field and the
             Unfeasible Routes Length field.

             Reachability information is encoded as one or more 2-tuples of
             the form <length, prefix>, whose fields are described below:

                      +---------------------------+
                      |   Length (1 octet)        |
                      +---------------------------+
                      |   Prefix (variable)       |
                      +---------------------------+

             The use and the meaning of these fields are as follows:

             a) Length:

                The Length field indicates the length in bits of the IP
                address prefix. A length of zero indicates a prefix that
                matches all IP addresses (with prefix, itself, of zero
                octets).

             b) Prefix:

                The Prefix field contains IP address prefixes followed by
                enough trailing bits to make the end of the field fall on an
                octet boundary. Note that the value of the trailing bits is
                irrelevant.

The minimum length of the UPDATE message is 23 octets -- 19 octets for the fixed header + 2 octets for the Unfeasible Routes Length + 2 octets for the Total Path Attribute Length (the value of Unfeasible Routes Length is 0 and the value of Total Path Attribute Length is 0).

An UPDATE message can advertise at most one route, which may be described by several path attributes. All path attributes contained in a given UPDATE messages apply to the destinations carried in the Network Layer Reachability Information field of the UPDATE message.

An UPDATE message can list multiple routes to be withdrawn from service. Each such route is identified by its destination (expressed as an IP prefix), which unambiguously identifies the route in the context of the BGP speaker - BGP speaker connection to which it has been previously been advertised.

An UPDATE message may advertise only routes to be withdrawn from service, in which case it will not include path attributes or Network Layer Reachability Information. Conversely, it may advertise only a feasible route, in which case the WITHDRAWN ROUTES field need not be present.

Next: 4.4 KEEPALIVE Message Format

Connected: An Internet Encyclopedia
RFC 1771 - 4.3 UPDATE Message Format